Polarization mode dispersion is one of the major types of dispersion which impairs the operation of fiber optical systems, especially systems with bit rates of 40 Gb/s or higher.
Polarization mode dispersion may be caused by many forms of imperfections along a fiber span. Asymmetry in the cylindrical form of the core or cladding, mechanical stress and other factors can cause polarization modes of the light passing through the fiber to propagate at different speeds depending upon their axes of alignment. This effect occurring at numerous different locations, orientations and magnitudes along a fiber span causes a spreading of optical pulses and a change in their shape.
Polarization mode dispersion is also wavelength dependent. A profile of PMD versus angular frequency ω reveals moreover that this relationship is not linear. As frequency changes, so does PMD, sometimes in a very non-trivial way. A functional form can be approximated near a particular carrier frequency ω0 by a functional expansion around ω0. In this way PMD is a combination of first order and higher order effects. First order PMD is a first order contribution based on the linear term, while second order PMD is based on the parabolic term in ω. The delay between polarized energy associated with first order PMD is called the differential group delay (DGD) and is characteristic of the delay between principal polarization states of the first order PMD. Nonprincipal polarization states undergo a dispersion which varies linearly with ω. Second order PMD is characterized by dispersion which All varies parabolically with ω. In WDM optical systems involving more than one optical frequency, measuring higher order PMD and compensating for it is important to maintain high bit-rate, and network operability.
The standard way to describe PMD is to express it as a three dimensional vector in Stokes Space. Each vector in Stokes Space represents a single unique polarization state. Every possible polarization state may be represented by a single vector in Stokes space the components of which are S1, S2, and S3, corresponding respectively to the amount of horizontal polarization, the amount of linear polarization at a 45 degree angle, and the amount of left-handed circular polarization. The convention for the sign or direction of the PMD vector is that it points in the direction corresponding to the polarization states in which the dispersion is positive. This convention is chosen for both the PMD of optical signals and for the PMD of compensation modules, although any convention which is consistent would suffice.
Tunable dispersion compensation is one of the key requirements for high bit rate fiber optical systems, especially for systems with bit rates of 40 Gb/s or higher. In modern optical communications systems, in addition to first order PMD compensation, sometimes second order PMD compensation is also needed. Currently compensators provide first order PMD compensation, or a combination of first and higher order PMD compensation. For a more detailed discussion regarding second order PMD and the need for its compensation, please refer to P. Ciprut, B. Gisin, N. Gisin, R. Passy, et al, “Second-order Polarization Mode Dispersion: Impact on Analog and Digital Transmissions,” IEEE J. Lightwave Technology, Vol.16(5), pp.757, 1998; Q- Yu, L. -S. Yan, Y. Xie, M. Hauer, A. E. Willner, “Higher Order Polarization Mode Dispersion Compensation Using a Fixed Time Delay Followed by a Variable Time Delay,” IEEE Photonics Technology Letters, Vol.13(8), pp.863, 2001.
A compensator which provides only second order PMD compensation with zero first order PMD compensation is desirable if, for example, first order polarization mode dispersion is already employed and an optical signal needs only second order polarization mode dispersion compensation. As such a second order PMD compensator enables the system to be separately optimized on both the first order PMD compensation and the second order PMD compensation. In order for the system to be separately optimized as described above, the second order PMD compensator should not change the first order polarization mode dispersion of the optical signals.